Ferromagnetic material



amazes! Patented Feb. 6, 1962 of Delaware No Drawing. Filed Oct. 14, 1957, Ser. No. 689,740 Claims priority, application Netherlands Oct. 19, 1956 5 Claims. (Cl. 25262.5)

The ferromagnetic materials, according to the invention. consist of crystals or mixed crystals of compounds having a chemical composition according to the formula where Me represents at least one of the bivalent metals from the series comprising Fe, Ni, Zn and Mg and where which compounds have a rhombohedral crystal structure the lementary cell of which can be described in the hexagonal crystal system by a c-axis of about 113.1 A. and an a-axis of about 5.9 A. A small part of ferric ions (Fe also can be replaced by trivalent chromium Cr and aluminum -(Al ions.

These materials have valuable electrical and magnetic properties. For example, they exhibit a saturation magnetisation of the same order of magnitude as the ferromagnetic ferrites having the crystal structure of the mineral spinel, the so-called spinel structure. Like most of these ferrites, most of the materials in accordance with the invention have a high specific resistivity. Thus these materials can be fabricated into cores for use at high frequencies, in many cases at frequencies of 200 mc./ sec. and more, since the high specific resistivity gives rise to small losses.

Unlike the ferromagnetic ferrites having a spinal structure, the initial permeability, for many of these materials is constant up to much higher frequencies. Since the use of ferromagnetic cores in a frequency range in which the initial permeability is not constant generally involves the occurrence of high electro-magnetic losses, the novel materials can be used as ferromagnetic bodies up to the above-mentioned higher frequencies in all cases where low electro-magnetic losses are required.

In ferromagnetic materials of hexagonal crystal structure, the crystal anisotropy is given as a first approximation by the expression (see R. Becker and W. Doring, Ferromagnetismus, 1939, page 114). If K is positive (so-called positive crystal anisotropy), the hexagonal axis of the crystal is the preferred direction of magnetisation. negative (which condition will hereafter be referred to as negative crystal anisotropy), this means that the spontaneous magnetisation is directed at right angles to the hexagonal axis and consequently parallel to the basal plane of the crystal. In the latter case, the crystal has a so-called preferred plane of magnetisation (there may, however, still be a comparatively weak preference of the magnetisation for certain directions in the basal plane). In part of the novel materials under consideration, K is negative. In this event, in each crystal the direction of the spontaneous magnetisation lies in the basal plane and in this plane the direction of magnetisation is more read- If, however, K is 6 ily rotatable than in a direction which does not lie in this plane. In these materials, the initial permeability has values which are high enough to be of interest for electromechanical use. This initial permeability is constant up to a far higher frequency than in ferromagnetic ferrites of spinel structure having an equal value of the initial permeability at low frequency. The materials in accordance with the invention having positive crystal anisotropy offer novel possibilities of manufacturing, for example, ferro-magnetic bodies having permanent magnetic properties and ferro-magnetic bodies for use in microwave-apparatus.

In order to determine whether or not crystals having a preferred direction of magnetisation or crystals having a preferred plane of magnetisation are involved, use may be made of the following identifications test:

A small amount, for example 25 mgs., of the crystal material to be examined is mixed as a fine powder with a few drops of a solution of an organic binder or adhesive 'in acetone, the mixture being spread on a glass slide. This slide is arranged between the poles of an electromagnet so that the lines of magnetic force are at right angles to the surface of the slide. By gradually increasing the electric direct current of the electromagnet, the magnetic field strength is increased, so that the powder particles rotate in the field in a manner such that either the preferred direction of the magnetisation or the preferred plane of the magnetisation becomes substantially parallel to the direction of the lines of magnetic force. By proceeding carefully, a coagulation of the powder particles can be avoided. After evaporation of the acetone, the powder particles adhere to the glass surface in a magnetically orientated condition. By means of radiographs it can now be determined which orientation of the powder particles has been produced by the action of the magnetic field. This can be effected, for example, with the aid of an X-ray diflractometer (for example, an apparatus as described in Philips Technical Review, 16, pages 123- 133, 1954l955). An increased occurrence of the reflections at planes at right angles to the hexagonal c-axis (so-called OOI-refiections) being observed in the case of a preferred direction parallel to the c-axis as compared with a radiograph of a nonoriented specimen. In the case of a preferred plane at right angles to the hexagonal c-axis, an increased occurrence of reflections at planes parallel to this c-axis (so-called hkO-refiections) is observed.

The crystal anisotropy constant K, deepnds upon the chemical composition and also for each material, upon the temperature. In many of the novel materials under consideration a temperature can be determined below which the crystal anisotropy is negative and above which it is positive. The temperature of the reversing point of the crystal anisotropy depends mainly upon the content of bivalent cobalt. The reversal point lies at room temperature, if the material contains a certain amount of Co, which amount slightly depends upon the other small bivalent ions but is about /3 of the total amount of small bivalent ions. If more than about /3 of the small bivalent ions is Co (d greater than or about equal to 0.6), the crystals of the materials have a preferred plane of the magnetisation at room temperature. If less than about /a of the small bivalent ions is Co, (a' is less than or about equal to 0.6), the crystals of the materials have a preferred direction of the magnetisation at room temperature. Obviously, the choice of the material is determined by the fact Whether a positive or a negative crystal anisotropy is desired within the operating range.

It has been found that in materials which have a negative crystal anisotropy constant and consequently a preferred plane of the magnetisation, the value of the initial permeability can be further increased if in the formula 3 ferric-ions are partly replaced by trivalent Co-ions, but this is only possible to about 1.5 atomic percent expressed by 0.2g0.5. Comparatively high values of the initial permeability are also found in the materials containing Fe as the bivalent metal; however, these materials naturally have a low specific resistivity which involves the production of eddy-current losses at high frequencies.

The production of the materials according to the invention is preferably eifected by heating (sintering) a finely powdered mixture of the component metal oxides of the novel compounds in about correct proportions. Obviously, at least one of the component metal oxides can be wholly or partially replaced by compounds which can be converted into metal oxides by heating, for example carbonates, oxalates and acetates. The component metal oxides can also be wholly or partially replaced by one or more reaction products of at least two of the component metal oxides, for example BaFe O The term correct proportions as used herein is to be understood to mean proportions of the amounts of the metals in the initial mixture equal to those in the materials to be produced. When using this method of producting the materials in accordance with the invention, the materials which do not contain Sr, Pb or Ca are produced by far the most readily.

If desired, the finely powdered initial material may be presintered, the reaction product being ground again and the powder thus obtained being re-sintered, and this sequence of operations may be repeated once or several times. This mehod of sintering is known per se, for example in the production of ferro-magnetic ferrites having spinel structure (see for example J. I. Went and E. W. Gorter, Philips Technical Review, 13, page 183, 1951-1952). The temperature of the sintering or final sintering process is between about 1000 C. and about 1450 0., preferably between 1200 C. and 1350 C.

'In order to promote the sintering process, sintering agents, for example silicates and fluorides, may be added. Bodies consisting of the above-described ferromagnetic materials can be formed either by immediately sintering the initial mixture of the metal oxides or the like in the desired shape or by pulverizing the reaction product of the presintering process and shaping it, if required after the addition of a binder, into the desired form, after which it may be re-sintered or hardened.

By sintering at a temperature materially exceeding 1200 C. and/or by sintering in a gas-atmosphere containing comparatively little oxygen, a material can be produced having a comparatively high proportion of Fe so that the specific resistivity can be reduced to values of less than ohm-cm. If this is not desired, because the material is to be used in magnetic cores for use at high frequencies without the production of excessive eddy-current losses at those frequencies, excessive formation of ferrous ions must be avoided or any excessive amount of ferrous ions must subsequently be oxidized to ferric ions in known manner, for example by heating again in oxygen at a temperature between 1000 C. and 1250 C.

In producing the materials in accordance with the invention which contain lead, special precautions must be taken. Part of the PbO escapes from the product during heating due to its volatility, so that it is desirable for the initial mixture to contain an amount of lead in excess of the proportions of the metals in the material to be produced.

As is usual, the electromagnetic losses are indicated by a loss factor tan =,u/ (see I. Smit and H. P. I. Wijin Advances in electronics, VI, 1954, page 69, formula No. 37). The quantity ,u' is the so-called real part of the initial permeability. This property and tan 6 will be given as numerical values in the following examples.

EXAMPLE I A mixture of BaCO CoCO and Fe O in proportions of 2 mols of BaCO 1 mol of CoCO and 9 mols of Fe O which corresponds to the desired compound Ba Co Fe O was mixed with ethyl-alcohol in a ball mill for half an hour. After drying, the mixture was pre-fired in air at 1000" C. for 15 hours, and the reaction product then ground together with ethyl alcohol in a ball mill for 1 hour. After drying, at small amount of a solution of an organic binder was added to the product and part of this mixture was compressed into a tablet which was fired in oxygen at 1260 C. for 1 hour.

In like manner, tablets were produced from mixtures comprising, in addition to BaCO and Fe O COCO3 and MgO, COCO3 and MgO, MgO, NiO and ZnO, respectively in a ratio of 2 mols of BaCO and 9 mols of Fe O to 0.5 mol of Coco and 0.5 mol of MgO, 0.25 mol of CoCO and 0.75 mol of MgO, 1 mol of MgO, 1 mol of NiO and 1 mol of ZnO, respectively, corresponding to the desired compounds Ba CoMgFe O a oe rs se sm 4 2 36 60, 4 2 36 60 and Ba Zn Fe O respectively.

Finally, a mixture of BaCOg, ZnO and Fe O in a ratio of 2 mols of BaCO 0.75 mol of ZnO and 9.125 mol of Fe O which corresponds to the desired compound was prepared, pre-fired and ground in the above described manner. A tablet was pressed from this mixture and fired at 1300" C. in technical nitrogen for 1 hour.

Examination by means of X-rays showed that the reaction product consisted substantially entirely of crystals having the desired structure. The saturation magnetisation of all these materials exceeded 2000 gauss. The sign of the crystal anisotropy at room temperature was also determined by X-ray examination of the materials in the manner referred to hereinbefore. The results are shown in Table 1. In this and the following tables the column captioned main constituent denotes chemical formulae which were deduced from the composition of the initial mixtures and from the X-ray examination.

Table No. 1

Crystal anisotropy at room temperature of a number of materials according to the invention.

Sign of the crystal anisotropy EXAMPLE II A mixture of BaCO CoCO and Fe O in a ratio according to the formula Ba Co Fe O was mixed with ethyl alcohol in a ball mill for half an hour and subsequently pre-fired in air at 1000 C. for 15 hours. The reaction product was ground together with ethyl alcohol in a ball mill for 1 hour and after drying and the addition of a small amount of an organic binder the product was compressed to form rings. One ring was fired in air at 1280 C. for 1 hour and subsequently slowly cooled in air. The properties of this ring are given in Table 2 under 1. Another ring was fired in oxygen at 1270 C. for 3 hours. The properties of this ring are given in Table 2 under 2. A third ring was fired in oxygen at 1270 C. for three hours, cooled at room temperature and subsequently heated again in oxygen at 1180 C. for 4 hours. The properties of this third ring are given in Table 2 under 3. Examination by means of X-rays showed that all the reaction products consist entirely of crystals having the desired structure. The measurements given in this and the following tables were obtained from rings in the demaguetised condition at room temperature according to the method described by C. M. van der Burgt, M. Gevers and H. P. J. Wijn in.

Philips Technical Review, 14 pages 246-252 (1952- 1953).

In Table 2 and the following tables, columns designated Q-cm., ,0 and tan 6 designate specific resistivity, initial EXAMPLE IV A mixture of BaCO CoCO and Fe O in a ratio corresponding to the formula (real) permeability and loss factor, respectively, at room 5 B 0 H m temperature of the materials according to the invention. emCoMom Initial permeability and loss factor were also measured was mixed with ethyl alcohol in a ball mill for half an at frequencies of kc./sec., 80 mc./sec., 260 mc./sec. hour and subsequently prefired in air at 1000 C. for and 500 rr'1c./sec. hours. The reaction product was ground with ethyl Table 2 10 kc./s. 80 mc./s. 260 mc./s. 500 mc./s. N0. Main constituent -9 -cm. Y

p p tan 6 4' tan 5 n tan 3 1 Ba Co1Fe3 O0 10 4.1 4.4 0.05 4.3 0.14 4.3 0.25 2 EMU/0117835000.. 10 3.1 2.7 0.01 2.9 0.07 2.9 0.19 3 Ba1C01Fe:oOo0..-- 10 as 3.1 0. 01 3.3 0.04 3.5 0.24

EXAMPLE III alcohol in a ball mill for 1 hour and, after drying and A mixture of 1321003 cocosand F6203 in a ratio the addition of a small amount of an organic binder, responding to the formula Ba Co Fe O was mixed g ff 1 533 5 3 'g TQfile ring with ethyl alcohol in a ball mill for half an hour. After a d g; 4 a 1 Z g' drying and the addition of a small amount of an organic 2 l gf, E 3 1 g t er rmg binder, part of the product was compressed into a ring was E m an a e 0 room and fired in oxygen at 1260 C. for 1 hour. The ring tempsmture and tslubsequently eated.agam m.oxygen at was subsequently cooled to room temperature and again for i The prop.ertle of thls nng are heated in oxygen at 1180 C. for 4 hours. In the same lsted un er 2 Tab e th1rd.rmg pressed manner, rings were produced from mixtures of BaCO i g i g g g fi fi i Coco and Fe O in ratios corresponding to the formulas yg a or Sn i y 000 e a room temperature and again heated 1n oxygen at 1180 Barc emOo o C. for 4 hours. Finally it was cooled in oxygen to room 2 0.1 I I I and temperature. The properties of this ring are listed under BmooyFmjoognaow 3 in Table 4. X-ray examination revealed that all the X-ray examination showed that all the reaction products reaction products consisted entirely of crystals having the desired structure.

Table4 10 kc./s. 80 mc./s. 260 mc./s. 500 mc./s. N 0. Main constituent fl -cm.

p 1' tan 6 1; tan 6 4 tan 6 1 narooyr'emoolflow 3.10 11.0 10.4 0.10 9.2 0.20 3.3 0.00

2 naroog remoofiflom 0.10 7.2 0.7 0.00 6.8 0.10 0.0 0.20

0 naiool reaauoolfiow 10 3.7 3.3 0.01 3.0 0.04 3.8 0.25

EXAMPLE V consisted entirely of crystals having the desired structure.

Furthermore a ring was produced in the manner described hereinbefore from a mixture of BaCO CoCO and Fe O in a ratio according to the formula and this ring was shown by X-ray examination to consist substantially entirely of crystals having the desired structure.

The specific resistivity of all these products exceeds 10 ohm/ cm. Further properties of these rings are given in Table 3.

A mixture of BaCO C0CO and Fe O in a ratio corresponding to the formula BarCogFeai 000E000 Table 3 10 kc./s. mc./s. 260 mc./s. 500 mc./s. Main constituent [4' tan 6 .1 tan 5 [2' tan 6 BmCOzFeasOuo 3. 8 3. 1 0. 01 3. 3 O. 04 3. 6 0. 24 Baloo lremoolffow a. 7 a. 3 0. 01 a. 0 0.04 3.8 0. 25

Ba coPFeamcoggOm 4. s 4. 3 0. 01 4. 4 0. 04 4. 0 0. 2s

B111'o0Fe0.5005fow. 4. 9 4.4 v 0. 01 4' 4 0. 04 a. 1 0. 22

1 211002 2110.tFemCofiOm, B34001.rMEMFBsMCOEEOm,

atcolf N mFeas.rCoffiOeo, BarCo ,Mno.EeamCoREOw X-ray examination showed that the reaction products '10 consisted entirely of crystals having the desired structure. The properties of the rings are given in Table 5.

a is at most 1, b is at most 0.7, c is at most 0.6, d is at most 2, e is at most 0.6, f is at most 0.6, g is at most 0.5, said crystals having a rhombohedral structure, the elementary cell of'which can be described in the hexagonal crystal system by a c-axis of about 113.1 A., and an axis of about 5 .9 A.

2. A ferromagnetic material consisting essentially of crystals having the composition:

in which Me is at least one bivalent metal selected from the group consisting of Fe, Ni, Zn and Mg and in which Table 5 kc./s. 80 mc./s. 260 trials. 500 rnc.ls. Main constituent 14 FL tan 6 p 1 tan 5 a tan 6 natooyremcogfgow 5. s 5. a 0. 03 5. 2 0.06 5. 3 0.15 narooigznmreat.tcofgom 6.0 6. 7 0. 03 6. 8 0.14 e. 4 0. 51 BmoofiM o.aremcofgow 5. 3 s. o 0.06 4. s 0.14 4. a 0. 2s natcoigrno,tremoogfgow 5. 3 5. 2 0. 03 s. s 0.18 4. 7 0. so BatOoif MnuAFB COifiOfli 4. o 3.6 0. 01 3. s 0. 04 a. s 0.16 natootgouurettlcofgow 3. 4 a. 0 0. 01 3.1 0. 04 a 1 0.20

EXAMPLE VI 0 is at most 1, b is at most 0.7, c is at most 0.6, d is at The compounds oc 'oz tz m t].85 0.15 12 l9 and Ba Ca Fe O were produced by heating mixtures of BaCO Fe O and one of the compounds SrCO PbCO and CaCO respectively, in ratios corresponding to those compounds at 1-000 C. for 15 hours. From these compounds and BaCO C000 and ZnO, mixtures were made in ratios of 3 mols of (Pa, Sr, Pb, Ba) -Fe O 1 mol of BaCO 1.5 mol of Coco and 0.5 mol of ZnO, corresponding to the desired compounds at 'u.e rs os ss so a.ss oAs i.5 0.5 se eo and ass oAa l .s os se sa To the mixture containing lead, 5% by weight of PbCO was added. The mixtures were ground with ethyl alcohol in a ball mill for 1 hour. After drying and the addition of a small amount of a solution of an organic binder, rings were pressed from the products, which were fired at 1260 C. in oxygen for 1 hour with the exception of the ring containing lead, which is fired at 1240 C. in oxygen most 2, e is at most 06, f is at most 0.6, g is at most 0.5, said crystals having a rhombohedral structure, the elementary cell of which can be described in the hexagonal crystal system of a c-axis of about 113.1 A., and an a-axis of about 5.9 A., said crystals having at room temperature a preferred plane of magnetization.

3. A ferromagnetic material consisting essentially of crystals having the composition:

in which Me is at least one bivalent metal selected from the group consisting of Fe, Ni, Zn and Mg and in which a is at most 1, b is at most 0.7, c is at most 0.6, d is at least about 0.6 and at most 2, e is at most 0.6, f is at most 0.6, g is at least 0.2 and at most 0.5, said crystals having a rhombohedral structure, the elementary cell of which can be described in the hexagonal crystal system 12y eke-axis of about 113.1 A., and an a-axis of about 4. A ferromagnetic material consisting essentially of crystals having the composition:

. 'n a l'sted in for 1 hour. The properties of the n gs re 1 Bawmms ruPbbCacMe, d r0 Cog CuiIMnIlFe/gfl) 00151060 Table 6.

Table 6 10 kc.ls. 80 mc./s. 260 mc./s. 500 mc./s.

Main constituent p p tan 5 1- tan 5 4' tan 6 B83,4Sfn s001 5Z110 5F83a0s0 61 8 3 05 7 0. 13 6. 5 O. 44 Bat.isP MsOOmZDo.sFGzaOau- 3. 8 3. 4 0. 04 3. 5 0. 13 3. 2 O. 33 Baa.55080.45C01.5ZI10-5F63t0n0 7 2- 5 0- 01 2. 6 0. 10 2. 5 0. 28

While we have thus described our invention in connection with specific examples and applications thereof we do not wish to be limited thereto as other embodiments will be readily apparent to those skilled in this art. The invention is defined in the appended claims which should be as liberally construed as permissible in view of the art.

What is claimed is:

1. A ferromagnetic material consisting essentially of crystals having the composition:

in which Me is at least one bivalent metal selected from the group consisting of Fe, Ni, Zn and Mg and in which in which Me is at least one bivalent metal selected from the group consisting of Pe Ni, Zn and Mg and in which a is at most 1, b is at most 0.7, c is at most 0.6, d is at most 0.6, e is at most 0.6, f is at most 0.6, g is at most 0.5, said crystals having a rhombohedral structure, the elementary cell of which can be described in the hexagonal crystal system by a c-axis of about 113.1 A., and an a-axis of about 5.9 A., said crystals having at room temperature a preferred direction of magnetization.

5. A ferromagnetic material consisting essentially of crystals having the composition:

9 10 in which Me is at least one bivalent metal selected FOREIGN PATENTS from the group consisting of Pe Ni, Zn and Mg and in 78,889 Netherlands Aug 15' 1955 which a is at most 1, b is at most 0.7, c is at most 0.6, 661,721 Great Britain Nov. 28 1951 d is at most about 0.6, e is at most 0.6, f is at most 0.6, 751,623 Great Britain July 4, 1956 g is at most 0.5, said crystals having a rhombohedral 5 165,447 Australia Oct 4, 1955 structure, the elementary cell of which can be described in the hexagonal crystal system by a c-axis of about OTHER REFERENCES 113.1 A. and an a-axis of about 5.9 A., said crystals Snoek: Physlca, June 1936, page 481. haymgat room temperature a preferred direction of mag Jonker et a Philips Tech' Rev. Nov 30, 1956' pp. netization. 10 145453 References Cited in the file of this patent Went et al.: Philips Tech. Rev., January 1952, p. 197. UNITED STATES AT N S 132122111111: ll hygical Rgvfilsetpt. 115,E19 PP- #7884338. 2,736,708 Crowley et a1 Feb. 28, 1956 a g 1 3 7 ngmeers' apan' 2,762,777 Went et al Sept. 11, 1956 15 2,778,803 Crowley Jan. 22, 1957 

1. A FERROMAGNETIC MATERIAL CONSISTING ESSENTIALLY OF CRYSTALS HAVING THE COMPOSITIONS: 